In order to produce the blown film, the resin must first flow into an annular path. Existing annular dies have a port in one side for entrance of the resin. These dies comprise an outer cylinder or circular die, a central mandrel and a bushing blocking one end of the die and having a slanted or helical profile directing the resin upwardly toward the exit of the die while guiding the resin circumferentially around the inner surface of the die. As designed, the gap between the circular die and mandrel is uniform to insure that the thickness of the flow paths are constant. The length of the path can be short, for flow going directly axially from the entrance port, while it must be very long for flow that passes to the opposite side of the mandrel before going axially. The experience with existing dies has been that the flow is not uniform, nor will the pressure be in any plane parallel to the die exit. Pressure and flow will be highest on the side of the entry port and lowest on the opposite side.
After travelling annularly within the die, the resin is directed through a metering gap formed by a flaring of the mandrel proximal the exit of the circular die. As the plastic resin exists, the pressure of internal air admitted through the center of the mandrel forms a bubble, the diameter of which will be several times the diameter of the die orifice. Thickness of the film will be nonuniform as will be its strength if the flow and pressure of resin through the die have been nonuniform.
Although this system is in common use, one problem that has remained is the uniform flow, pressure and distribution of the plastic resin within the die prior to formation of the bubble. Inasmuch as the resin greatly prefers axial flow, immediately adjacent the side port, to the circumferential flow around the mandrel, one approach has been to provide a large annular flow path followed by a restricted axial flow path, which system can be repeated several times within the die to reduce the foregoing differences in uniformity. The resin path can also be structured spirally, further reducing pressure and flow differences.
While these means will reduce the differences, they cannot eliminate them. Further correction has been facilitated by employing a movable bushing at the die exit which can be adjusted to lessen the gap near the entry port and increase it in the area furthest therefrom. As is understandable, these means represent increases in equipment costs as well as labor where adjustments are necessitated.
Pressure and flow differences also occur where two or more concentric layers of material are to be simultaneously extruded from concentric dies. Each successive layer will be nonuniform in thickness and will displace the mating layers where its pressure is greatest and, in turn, will be displaced where the other layer or layers are at maximum pressure. It is not feasible to provide movable bushings for the inner layer or layers inasmuch as the bushing and adjustment means are totally within the surrounding layers.
Multi-layered products produced by a multiple-bubble approach are becoming more desirable for items such as garbage bags, trash liners and food wrapping materials. It is therefore highly desirable that a die system be developed that will facilitate the manufacture of such films. No existing approach for single or for multi-layered films of which I am aware provides a uniform flow and pressure of the resin without special, complex tooling of the die and accompanying bushings and then, uniformity does not always result.